Titular metropolis in Macedonia. It was at first a village called Alia, situated not far from Axius, the modern Vardar; it subsequently took the name of Therma, from the thermal springs east and south of it. The gulf on which it was situated was then called the Thermaic Gulf. After having sheltered the fleet of King Xerxes and having belonged to the Athenians during the Peloponnesian War, Therma passed to the kings of Macedonia after the death of Alexander. Cassander, the son of Antipater, having enlarged the village and transported thither the inhabitants of the neighbouring villages, called it Thessalonica, in honour of his wife. Thenceforth the city grew steadily in importance. Unsuccessfully besieged by Æmilius Paulus, it only opened its gates after the victory of Pydna which made the Romans masters of Macedonia (168 B.C.). The kingdom was then divided into four districts, each of which had its capital and its conventus. Thessalonica was the capital of the second district. In 146 B.C. Macedonia was made a single province with Thessalonica as capital. This was the arrangement until the third and fourth century of our era, when four provinces were again formed. The proconsul had his residence at Thessalonica, as did later the prefect of Illyricum Orientale, who first resided at Sirmium. During the first civil war Thessalonica was the principal headquarters of Pompey and the Roman senators; during the second it supported Anthony and Octavius against the Triumvirs, receiving from them after the battle of Philippi the title of free city and other advantages, being allowed to administer its own affairs and obeying magistrates called politarchs.

Thessalonica received the title of colonia under the Emperor Valerian. Theodosius the Great punished the revolt of its inhabitants (390) by a general massacre in which 7000 were slain. In 479 the Goths attacked the city. Between 675 and 681 the Slavs unsuccessfully besieged Thessalonica four times. On 31 July, 904, a Mussulman corsair, Leo of Tripoli, came unexpectedly with his fleet and attacked the city, then the second in the empire, captured and pillaged it, and took away a great many prisoners. A dramatic account of the affair was written by a priest of Thessalonica, John Cameniates, who was an eyewitness (Schlumberger, "Nicéphore Phocas", Paris, 1890, 35 sqq.). In 1083 Euthymius, Greek Patriarch of Jerusalem, was commissioned by Alexius I Commenus to negotiate peace at Thessalonica with Tancred of Sicily, who had conquered a portion of Epirus and Macedonia and threatened to take possession of the rest. In August, 1185, Guillaume d'Hauterive, King of Sicily, besieged Thessalonica by sea with a fleet of 200 ships and by land with an army of 80,000 men; the city was captured, and all resistance from the Greeks punished with death. In the following year the city was recaptured by the Byzantines; the metropolitan Eustathius wrote an account of the campaign in a homily, which was read during the Lent of 1186. In 1204, after the Latins had occupied Constantinople and a portion of the Byzantine Empire, Boniface, Marquis of Monferrato, proclaimed himself King of Thessalonica, his Latin Kingdom depending on the Latin Empire of Byzantium. He defended it against the Bulgars, whose tsar, the terrible Calojan, was assassinated under the walls of Thessalonica in 1207, and against the Greeks from Epirus. In 1222 the latter put an end to the Frankish Kingdom and took possession of Thessalonica, setting up an independent empire, the rival of that of Nicaea, with Theodore Comnenus as first sovereign. He was defeated in 1230 at Klokotinitza by the Bulgar Tsar, Assen II, and most of empire passed into the hands of the Bulgars. Thessalonica with the remaining cities was given to Theodore's brother, the Emperor Manuel.

In 1242 after a successful campaign against the Emperor of Thessalonica, John Vatatzes, Emperor of Nicaea, forced John Angelo to take only the title of despot and to declare himself the vassal. After the expedition of Vatatzes in 1246 Thessalonica lost all independence and was annexed to the Empire of Nicaea which in 1261 was once more removed to Constantinople. Unable to defend it against the Turks, the Greeks in 1423 sold Thessalonica to the Venetians, the city being captured 28 March, 1430, by the Sultan Murad and definitively incorporated in the Ottoman Empire. It was the scene of unheard-of-cruelties on the part of the Turks. In order to weaken the Greek element, so powerful in the city and in that part of Macedonia, the Sublime Porte offered a refuge about the end of the sixteenth century to the Jews driven from Spain by Philip II. They now number 80,000 out of 120,000 inhabitants; the remainder of the population consists of Turks, Greeks, Bulgars, Armenians, and nearly 3000 Catholics. The parish is directed by the Lazarists, the schools by the Christian Brothers. Thessalonica, which is the capital of a vilayet, grows constantly in importance, owing to its situation and its commerce, as well as to the part it played in the two military revolutions of 1908 and 1909 which modified the authoritative régime of the Turkish Empire.

The establishment of Christianity in Thessalonica seems to date from St. Paul's first journey to the city (see ). Secundus and Aristarchus, companions of St. Paul, were natives of Thessalonica (Acts, xx, 4); Demas who abandoned the Apostle to go thither, seems likewise to have been born there (II Tim., iv, 9). According to Origen, who repeats an ancient tradition ("Comment in Ep. ad Rom.", in P.G., XIV, 1289), Gaius was the first Bishop of Thessalonica. Four persons of this name are mentioned in the New Testament, but the Gaius of Origen would be a native of Corinth (I. Cor., i, 14). Melito of Sardes relates that Antoninus Pius wrote to the Thessalonians not to tolerate in their city the tumult against the Christians (Eusebius, "Hist. eccl.", IV, 26). Alexander assisted at the Council of Nicaea in 325, at Tyre in 335, and at the consecration of the Holy Sepulchre in the same year. At the end of the same century Acholius baptized Theodosius the Great. Le Quien has compiled a list of 74 Greek titulars of this city, some of whom do not belong to it. Father Petit continued his task and gives a biographical account of more than 130. The most famous were: Rufus, who in the early fifth century acted constantly as intermediary between the papacy and the Eastern Churches; Eusebius, the correspondent of St. Gregory the Great and author of a work in ten books against the Monophysites; John, who early in the seventh century compiled the first book on the miracles of St. Demetrius; St. Joseph, brother of St. Theodore the Studite, and the victim in 832 of the Iconoclast persecutions; Leo the Philosopher, professor at the Magnaura, the master of Photius and of all the literary celebrities of the period; Michael Chumnos, the author of several canonical treatises in the twelfth century; Basil of Achrida, who took part in the theological discussions with the envoys of the pope or of the Emperor of the West; Eustachius, the celebrated scholiast of Homer; Gregory Palamas, the defender of the Hesychast theories and the bitter enemy of the Catholics in the fourteenth century, who is still regarded as one of the greatest doctors of the Schismatic Church; Isidore Glabas; Simeon, liturgist and canonist, d. in 1429, a year before the capture of the city by the Turks.

When Illyricum Orientale, comprising the two civil dioceses of Dacia and Macedonia, was ceded by Gratian in 379 to the Empire of the East, Pope St. Damasus in order to retain jurisdiction over these distant provinces appointed the Bishop of Thessalonica his vicar Apostolic. In this capacity the bishop resided at the local councils of the various provinces, judging and solving difficulties, save in more serious matters, wherein the decision was reserved to the pope. He also confirmed the election of metropolitans and simple bishops and granted authorization to proceed to ordination. Finally, he occupied a privileged place at the oecumenical councils and signed their decisions immediately after the patriarchs. He thus enjoyed the prerogatives of a patriarch, even to bearing the title, but was subject to the Patriarch of Rome. The Bishop of Constantinople sought to modify this organization by inducing Theodosius II to pass a law (14 July, 421) which attached all the bishops of Illyria to the Byzantine Church, and by having this law inserted in the Code (439); but the popes protested against this injustice and prevented the application of the law. Until 535 the Vicar Apostolic of Thessalonica exercised jurisdiction over all the provinces of Illyricum Orientale, but subsequent to Novel xi of Justinian the authority was divided between him and the new Archbishop of Justiniana Prima. The latter, likewise appointed vicar Apostolic of the pope, directed the seven provinces of the north while the Bishop of Thessalonica continued to occupy the six others: Macedonia Prima, Thessalia, Achaia, Creta, Nova and Vetus Epirus. Matters remained so until 732 when the Emperor Leo the Isaurian, after his excommunication by the pope, connected all the bishoprics of Illyria with the Patriarchate of Constantinople. Thenceforth, despite the protests of Rome, Thessalonica was dependent on the Church of Byzantium.

After the establishment of the Latin Kingdom of Thessalonica in 1205 Nivelo de Chérisy, Bishop of Soissons, who had taken an active part in the Fourth Crusade, was appointed by Innocent III (10 December, 1206) first Latin archbishop of the city. He died in the following year; his successors were at first residential and afterwards titular (see list in Le Quien, "Oriens Christ.", III, 1089-96; Eubel, "Hierarchia catholica medii aevi", I, 510; II, 275). From a letter of Innocent III written in 1212 we learn that Thessalonica had then eleven suffragans. Apart from the saintly bishops mentioned above Thessalonica had other saints: Agape, Irene, and Chionia, martyred under Diocletian; Agothopodus, deacon, and Theodulus, rector, martyred under Diocletian; Anysia, martyred under Maximian; Demetrius, martyr, the protector of the city, from whose tomb flowed an oil which worked miracles, and whose superb basilica has been converted into a mosque; David, solitary (sixth century); Theodora, d. in 892; etc. The Vicariate Apostolic of Macedonia, for the Bulgars, whose titular resides at Thessalonica, was established in 1883. It has upwards of 6000 Catholics, 26 residential stations, 33 secular priests, most of them married, 10 Lazarist priests, 21 churches and chapels, 27 primary schools for boys and girls with 1110 pupils. The seminary, directed by the Lazarists, is at Zeitenlik, near Thessalonica. The Sisters of Charity and the Bulgarian Eucharistine Sisters also have schools and orphanages.